Heretofore, a lipid fraction derived from egg yolk and containing phospholipids, typically phosphatidylcholine (herein abbreviated to PC), namely, the so-called egg yolk lecithin, is widely used in the field of cosmetics or pharmaceutical products as an emulsifier or a liposome-forming agent as a carrier for drugs by virtue of the surface activity, permeating effect and other properties of the phospholipids contained therein.
It so happens that PC has the structure of cylindrical molecules comprising balanced sizes of hydrophobic and hydrophilic groups, and when egg yolk lecithin with a high PC content is used as a liposome-forming agent, it is therefore capable of forming a liposome having a stable lipid bilayer. If egg yolk lecithin with a high PC content and yet a low PE content is used in this case, it is possible to produce a liposome having higher structural stability, i.e., a strong and pliant membrane. When used as a starting material for cosmetics such egg yolk lecithin with a high PC content and a low PE content is also capable of stabilizing various physical properties of the end product.
In view of the above, if egg yolk lecithin with reduced PE content and relatively increased PC content could be produced, such egg yolk lecithin would be highly beneficial from a commercial point of view.
Egg yolk lecithin (lipid fraction) obtained by extraction from egg yolk by a conventional method essentially comprises phospholipids, such as PC and PE, and neutral lipids, such as triglyceride and cholesterol.
Various methods for fractionation of phospholipids from the egg yolk lecithin have heretofore been known in the art. For example, Japanese Patent Laid-Open Pub. No. 152392/1984 discloses a method wherein a solution of a lipid fraction dissolved in a non-polar or weakly polar solvent is brought into contact with a resin which adsorbs phospholipids preferentially to cuase the resin to adsorb the phospholipids, and thereafter the phospholipids thus adsorbed are eluted with a polar solvent. In accordance with this method, it is possible to obtain phospholipid fractions having different PE to PC ratios by collecting the eluates according to fractions. However, a fraction having a high PC content and a low PE content can be obtained only at an exceedingly low proportional rate (about 10%) for the starting lipid fraction to be treated, and yet copious quantities (about 200-fold (v/v) volume) of the resin is required for the starting material. It is therefore difficult to fractionate by this method phospholipids having a high PC content and alow PE content economically on a commercial scale.
Furthermore, various methods for increasing the PC component in egg yolk lecithin have been attempted so far. For example, solvent fractionation which comprises adding water to egg yolk lecithin dissolved in ethanol thereby to precipitate PE has been known in the art. This method, however, is accompanied by a problem in that an increase in PC content results in a notably low yield of the end product. As another example, a method using an adsorbent such as silica gel or alumina has been known in the art. This method, however, entails some difficulties when it is practiced on a commercial scale such as that the yield of the end product is low although PC and PE can be fractioned from each other effectively and that the adsorbent is required in great quantities. Still another example of such known methods comprises forming a metal salt complex with Cd, Ca, Mg, Zn or the like and utilizing the solubility thereof. However, this method is again accompanied by the problem of the metal remaining inevitably in the end product obtained.
Under such circumstances, a primary object of the present invention is to provide a process whereby egg yolk lecithin with reduced PE content and relatively increased PC content can be produced in a high yield on a commercial scale.